1. Field of the Invention
The present invention relates to compound archery bows, and particularly to eccentrics operable with such bows.
2. State of the Art
Compound archery bows employ a pulley system with bow string rigging arranged to provide a mechanical advantage to deflect flexible bow limbs, and to provide a draw force let-off at full draw. The limbs of a typical compound bow are much more stiff than limbs of a typical prior art single action bow, such as a recurve or long bow. Therefore, the limb deflection of a compound bow can be reduced while still storing sufficient energy to provide enhanced arrow speed compared to such prior art bows. The draw force let-off effected by the pulley arrangement permits an archer to hold an arrow at full draw with reduced exertion, likely resulting in more accurate shot placement than with a single action bow.
For purposes of this disclosure, brace, or a brace condition, is defined as the orientation achieved in a fully strung bow having tension applied to the drawstring solely by the bow limbs. That is, brace is defined as a static position of a bow that is ready to nock an arrow.
The term “pulley” encompasses a single wheel or eccentric element, but also includes an assembly of one or more such components. In the latter case, the term “pulley assembly” is sometimes used. The components that make up a pulley, or pulley assembly, are primarily wheels, or eccentrics. In an archery context, a wheel typically defines a groove, or string track, in which to receive a bow string rigging element, that is concentric with an axis of rotation of the wheel. An eccentric defines a groove, or string track, in which to receive a rigging element, that is spaced by a variable radius from the axis of rotation of the eccentric. Sometimes, an eccentric or wheel may be identified as a “cam” substantially in accordance with its ordinary dictionary meaning. However, in certain cases, principally for marketing language, a bow may be referred to in terms of selected characteristics of its pulley members. In marketing lingo, a pulley, or pulley assembly, may sometimes be referred to as a “cam”.
Bow string rigging for a compound archery bow is to be understood to encompass one or more two-force members that can be arranged to cause pulley rotation during a draw motion. One two-force member is adapted to serve as a drawstring. The drawstring may be a central, or intermediate, stretch of a longer string, or cable, that is entrained about one or more pulleys with ends of the cable being anchored to structure. End stretches of string rigging are typically referred to as cables, regardless of their actual construction. Modern practice typically provides drawstrings made from a multistrand, synthetic material, and end stretches made from other material, including aircraft cable, although any workable arrangement, or combination of materials is acceptable for practice of the invention. A stretch of cable having an end anchored to a limb, or other nonrotating structure, is typically classified as a power cable. A stretch of cable anchored between pulleys is sometimes called a control cable, although a drawstring may be similarly anchored. A stretch of cable may be regarded as a rigging element.
Early compound archery bows, such as disclosed in U.S. Pat. No. 3,486,495 to Allen, employed a pair of pulleys located for eccentric rotation disposed at tip ends of opposite bow limbs. Bow string rigging was entrained about the pulleys such that an end of a rigging element was anchored to each opposite bow limb. Such an anchor arrangement effectively provides two cable reference anchors to the bow. Maintaining timing of the two pulleys with respect to each other in such a string rigging arrangement is critical to achieving stable arrow flight. As the pulleys lose rotational synchronization with each other, the nocking point inherently departs from a straight-line path between full draw and a brace condition. Such nonlinear nocking point travel can cause erratic arrow flight, and loss of accuracy. It is common for a bow carrying such rigging to “go out of time”, due to any number of factors, such as cable stretch, or pulley slipping relative to the cable rigging. Archery bows having such rigging may be classified as “dual cam” bows for marketing purposes.
Several approaches have been proposed to overcome the timing problem associated with typical “dual cam” bows. Among more recent such attempts is an improved pulley system, often referred to as a “single cam” arrangement. McPherson, in U.S. Pat. No. 5,368,006 discloses a bow exemplifying such a configuration. The improved pulley arrangement places an eccentric cam element at only one limb end, and a cooperating idler cam element at the opposite limb end. Such an idler cam is concentric about its mounting axle, so the idler cam cannot effect timing of the opposite pulley. A single cable reference anchor is provided at the limb end carrying the idler. Synchronization between the pair of pulleys mounted on the bow is inherent due to the single eccentric element. Bows of this type may be regarded as true “single cam” bows. However, such true “single cam” bows also inherently force a transverse component in nocking point travel between full draw and brace. The eccentric cam element of one pulley unavoidably unwraps drawstring at a variable rate while the idler cam component of the opposite pulley unwraps drawstring at a constant rate. Therefore, the transverse nocking point travel is nonlinear between full draw and a brace condition in such a “single cam” bow. Such behavior is also evident in certain modified forms of the “single cam” assembly, especially if one, or both, pulleys included in the rigging is/are adjustable to change draw length of the bow.
It can be difficult to set up, or tune, a bow to provide consistent, straight arrow flight. As a first step, the timing between pulley assemblies may need to be adjusted to synchronize pulley rotation. Further adjustments may be required to the nocking point location on the drawstring, and to both lateral and vertical position of the arrow rest, to minimize wobble of an arrow in flight. Once a bow is set up, it can be frustrating if the pulley timing changes, as frequently occurs over time in certain known archery bows. Making an adjustment to the bow, such as changing the draw length, often compromises the tune of the bow by changing the timing between the pulley members. In the case of certain “one cam” bows, a change in draw length inherently causes an undesirable change in the nocking point travel path. A major problem with certain prior art bows is simply keeping rotation of the pulleys synchronized, while permitting a simple, easy adjustment in certain bow characteristics, such as draw length. One attempt to address this problem is disclosed by Larson in U.S. Pat. No. 4,774,927. Larson discloses a pulley having a rotatable cam portion, or module, operable to change a draw length of a bow on which the pulley is mounted.
Considerable effort has been devoted to developing pulley shapes to preserve a draw weight let-off while maximizing stored energy in a bow's limbs. Pulley shapes encompass the various string and cable grooves-carried on the individual cam elements forming the pulley assemblies. Miller, in U.S. Pat. No. 5,505,185, discloses certain desirable component elements of a pulley assembly, including a power cam element. It would be desirable further to provide an improved profile for pulley elements operable to better harness the stored limb energy for stable transfer of that energy to an arrow to increase certain shooting characteristics of a bow, such as arrow velocity.
End stretches of cables are often anchored to post-type structure carried on a pulley of bow string rigging, or on a component forming such a pulley. Commonly, a relatively short, stubby, post-type anchor is affixed to a cam component for anchoring a cable of an immediately adjacent cam component. In certain cases, an anchor may have a desired foundation location spaced apart, by one or more cam components, from a plane in which the anchored cable acts to apply loads to the anchor. Such circumstances require a tower anchor, which increases the moment arm by which cable loads are amplified with respect to the foundation. Often, cable loads on the anchor structure reach a peak value as an arrow is fired, and the brace cable load, plus an additional impact load, is resisted by the anchor. In some cases, the anchor desirably is arranged to be removable from its foundation, e.g. to replace or to install certain pulley components. In such cases, cable loads may cause failure of the foundation, or of the fastening arrangement used to affix the tower anchor to the foundation.
Prior art bows, in general, often display certain undesirable traits. One such trait is the undesirable “click” produced by rotation of a positive draw stop into interference with a rigging member. Such a click can alert a hunter's quarry to the hunter's presence. One commercially available solution adhesively affixes a dampener pad to a contacting surface of a cam-mounted draw stop surface. Such dampener pad is prone to loss by being scraped from the draw stop surface, or by loss of adhesion between the draw stop surface and the dampener pad.
Excessive vibration subsequent to release of an arrow is another undesirable trait of certain bows. In certain instances, pulleys having press-fit bearing assemblies “walk” or move transversely with respect to their bearing assemblies due to vibration and side load applied from bow string rigging. Sometimes, such pulleys displace or transversely “walk” sufficiently with respect to their mounting bearing that the pulley detrimentally rubs, or scrapes, on spacers or other structure associated with the pulley mounting area. It would be an improvement to provide bow rigging elements operable to address the deficiencies found in prior art archery bows.